As conventional compression ratio changing devices in internal combustion engines, there are known (1) a compression ratio changing device in which a piston outer element is threadedly fitted over an outer periphery of a piston inner element, so that the piston outer element is advanced and retracted relative to the piston inner element to a lower-compression ratio position and a higher-compression ratio position by rotating and reversing the piston outer element (for example, see Japanese Patent Application Laid-open No.11-117779), and (2) a compression ratio changing device in which a piston outer element is axially slidably fitted over an outer periphery of a piston inner element, and an upper hydraulic pressure chamber and a lower hydraulic pressure chamber are defined between the piston inner and outer elements, so that the piston outer element is operated to a lower-compression ratio position and a higher-compression ratio position by supplying a hydraulic pressure alternately to the hydraulic pressure chambers (for example, see Japanese Patent Publication No. 7-113330).
It should be noted here that in the device (1), in order to operate the piston outer element to the lower-compression ratio position and the higher-compression ratio position, it is necessary to rotate the piston outer element. For this reason, the shape of a top face of the piston outer element cannot be determined freely in correspondence to the shape of a ceiling surface of a combustion chamber and the dispositions of intake and exhaust valves, and it is difficult to sufficiently increase the compression ratio of the engine in the higher-compression ratio position. In the device (2), particularly when the piston outer element is in the higher-compression ratio position, a large thrust load received by the piston outer element in an expansion stroke of the engine is supported by a hydraulic pressure in the upper hydraulic pressure chamber and hence, a seal withstanding a high pressure is required in the upper hydraulic pressure chamber. Moreover, when bubbles are produced in the upper hydraulic pressure chamber, the higher-compression ratio position of the piston outer element is unstable and hence, it is necessary to provide a means for removing such bubbles and as a result, an increase in cost as a whole is inevitable.